meng 3324: manufacturing processes, utpb, ch13

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    2012 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 5/e

    SHAPING PROCESSES FOR

    PLASTICS

    1. Properties of Polymer Melts

    2. Extrusion

    3. Production of Sheet, Film, and Filaments4. Coating Processes

    5. Injection Molding

    6. Other Molding Processes

    7. Thermoforming

    8. Casting

    9. Polymer Foam Processing and Forming

    10. Product Design Considerations

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    Plastic Products

    Plastics can be shaped into a wide variety of

    products:

    Molded parts

    Extruded sections

    Films

    Sheets

    Insulation coatings on electrical wires

    Fibers for textiles

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    More Plastic Products

    In addition, plastics are often the principal ingredient

    in other materials, such as

    Paints and varnishes

    Adhesives

    Various polymer matrix composites

    Many plastic shaping processes can be adapted to

    produce items made of rubbers and polymer matrix

    composites

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    Trends in Polymer Processing

    Applications of plastics have increased at a much faster

    rate than either metals or ceramics in the last 60 years

    Many parts previously made of metals are nowmade of plastics

    Plastic containers have been largely substituted for

    glass bottles and jars

    Total volume of polymers (plastics and rubbers) nowexceeds that of metals

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    Plastic Shaping Processes are

    Important

    Almost unlimited variety of part geometries

    Plastic molding is a net shape process

    Further shaping is not needed

    Less energy is required than for metals due to much

    lower processing temperatures

    Handling of product is simplified during production

    because of lower temperatures

    Painting or plating is usually not required

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    Two Types of Plastics

    1. Thermoplastics

    Chemical structure remains unchanged during

    heating and shaping Comprises ~ 70% of total plastics tonnage

    2. Thermosets

    Undergo a curing process during heating and

    shaping, causing a permanent change in

    molecular structure (called cross-linking)

    Once cured, they cannot be remelted

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    Classification of

    Shaping Processes

    Extruded products with constant cross section

    Continuous sheets and films

    Continuous filaments (fibers)

    Molded parts that are mostly solid

    Hollow molded parts with relatively thin walls

    Discrete parts made of formed sheets and films

    Castings

    Foamed products

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    Polymer Melts

    To shape a thermoplastic polymer it must be heated

    so that it softens to the consistency of a liquid

    In this form, it is called apolymer melt Important properties of polymer melts:

    Viscosity

    Viscoelasticity

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    Viscosity of Polymer Melts

    Fluid property that relates shear stress to shear rate

    during flow

    Due to its high molecular weight, a polymer melt isa thick fluid with high viscosity

    Most polymer shaping processes involve flow

    through small channels or die openings

    Flow rates are often large, leading to highshear rates and shear stresses, so significantpressures are required in these processes

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    Viscosity and Shear Rate

    Viscosity of a polymer

    melt decreases with

    shear rate Thus the fluid becomes

    thinner (flows more

    easily) at higher shear

    rates

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    Viscosity and Temperature

    Viscosity

    decreases with

    temperature Thus the fluid

    becomes thinner

    at higher

    temperatures

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    Viscoelasticity

    Combination of viscosity and elasticity

    Example: die swellin extrusion - hot plastic expands

    when exiting the die opening (memory)

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    Extrusion

    Material is forced to flow through a die orifice to

    provide long continuous product whose cross section

    is determined by the shape of the orifice Widely used for thermoplastics and elastomers to

    mass produce items such as tubing, pipes, hose,

    structural shapes, sheet and film, continuous

    filaments, and coated electrical wire Carried out as a continuous process; extrudate is

    then cut into desired lengths

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    Extruder

    Components and features of a (single-screw)

    extruder for plastics and elastomers

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    Extruder Components

    Two main components of an extruder:

    1. Barrel

    2. Screw

    The die is not an extruder component

    It is a special tool that must be fabricated for the

    particular profile to be produced

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    Extruder Barrel

    Internal diameter typically ranges from 25 to 150

    mm (1.0 to 6.0 in.)

    L/D ratios usually ~ 10 to 30: higher ratios forthermoplastics, lower ratios for elastomers

    Feedstock fed by gravity onto screw whose

    rotation moves material through barrel

    Electric heaters melt feedstock; subsequentmixing and mechanical working adds heat which

    maintains the melt

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    Extruder Screw

    Divided into sections to serve several functions:

    Feed section - feedstock is moved from hopper and

    preheated Compression section - polymer is transformed into

    thick fluid, air mixed with pellets is extracted from

    melt, and material is compressed

    Metering section - melt is homogenized andsufficient pressure developed to pump it through dieopening

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    Die End of Extruder

    Progress of polymer melt through barrel leadsultimately to the die zone

    Before reaching the die, the melt passes through ascreen pack- series of wire meshes supported by astiff plate containing small axial holes

    Functions of screen pack:

    Filters out contaminants and hard lumps

    Builds pressure in metering section

    Straightens flow of polymer melt and removes its"memory" of circular motion from screw

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    Melt Flow in Extruder

    As screw rotates, polymer melt is forced to move

    forward toward die; as in an Archimedian screw

    Principal transport mechanism is drag flow, Qd,resulting from friction between the viscous liquid and

    the rotating screw surfaces

    Compressing the polymer melt through the die

    creates a back pressure that reduces drag flowtransport (called back pressure flow, Qb )

    Resulting flow in extruder is Qx= QdQb

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    Die Configurations and

    Extruded Products

    The shape of the die orifice determines thecross-sectional shape of the extrudate

    Common die profiles and corresponding extrudedshapes:

    Solid profiles

    Hollow profiles, such as tubes

    Wire and cable coating

    Sheet and film

    Filaments

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    Extrusion of Solid Profiles

    Regular shapes such as

    Rounds

    Squares

    Irregular cross sections such as

    Structural shapes

    Door and window moldings

    Automobile trim

    House siding

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    Extrusion Die for

    Solid Cross Section

    (a) Side view cross section of extrusion die for solid

    regular shapes, such as round stock; (b) front view of

    die, with profile of extrudate

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    Hollow Profiles

    Examples: tubes, pipes, hoses, and other cross

    sections containing holes

    Hollow profiles require mandrel to form the shape

    Mandrel held in place using spiderlegs

    Polymer melt flows around legs supporting the

    mandrel to reunite into a monolithic tube wall

    Mandrel often includes an air channel for blowing

    air to maintain hollow form of extrudate

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    Extrusion Die for Hollow Shapes

    Side view of extrusion die for hollow cross sections; Section A-A is

    a front view of how mandrel is held in place; Section B-B shows

    tube prior to exiting die; die swell enlarges diameter

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    Wire and Cable Coating

    Polymer melt is applied to bare wire as it is pulled at

    high speed through die

    A slight vacuum is drawn between wire and polymerto promote adhesion of coating

    Wire provides rigidity during cooling

    Usually aided by passing coated wire through a water

    trough Product is wound onto large spools at speeds up to 50

    m/s (10,000 ft/min)

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    Extrusion Die for Coating Wire

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    Polymer Sheet and Film

    Film - thickness below 0.5 mm (0.020 in.)

    Packaging - product wrapping material, grocery

    bags, and garbage bags Stock for photographic film

    Pool covers and irrigation ditch liners

    Sheet - thickness from 0.5 mm (0.020 in.) to about12.5 mm (0.5 in.)

    Flat window glazing

    Thermoforming stock

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    Materials for

    Polymer Sheet and Film

    All thermoplastic polymers

    Polyethylene, mostly low density PE

    Polypropylene

    Polyvinylchloride

    Cellophane

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    Sheet and Film

    Production Processes

    Most widely used processes are continuous, high

    production operations

    Processes include: Slit-Die Extrusion of Sheet and Film

    Blown-Film Extrusion Process

    Calendering

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    Slit-Die Extrusion

    of Sheet and Film

    Production of sheet and film by conventional

    extrusion, using a narrow slit as the die opening

    Slit may be up to 3 m (10 ft) wide and as narrowas around 0.4 mm (0.015 in)

    A problem is thickness uniformity throughout width

    of stock, due to drastic shape change of polymer

    melt as it flows through die Edges of film usually must be trimmed because of

    thickening at edges

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    Slit Die Extrusion

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    Blown-Film Extrusion Process

    Combines extrusion and blowing to produce a tube of

    thin film

    Process sequence:1. Extrusion of tube

    2. Tube is drawn upward while still molten and

    expanded by air inflated into it through die

    Air is blown into tube to maintain uniform filmthickness and tube diameter

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    Blown-film

    Process

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    Calendering

    Feedstock is passed through a series of rolls to

    reduce thickness to desired gage

    Expensive equipment, high production rates Process is noted for good surface finish and high

    gage accuracy

    Typical materials: rubber or rubbery

    thermoplastics such as plasticized PVC Products: PVC floor covering, shower curtains,

    vinyl table cloths, pool liners, and inflatable toys

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    Calendering

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    Fiber and Filament Products

    Definitions:

    Fiber- a long, thin strand whose length is at least

    100 times its cross-section Filament- a fiber of continuous length

    Applications:

    Fibers and filaments for textiles

    Most important application Reinforcing materials in polymer composites

    Growing application, but small relative to textiles

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    Materials for Fibers and

    Filaments

    Fibers can be natural or synthetic

    Natural fibers constitute ~ 25% of total market

    Cotton is by far the most important staple

    Wool production is much less than cotton

    Synthetic fibers constitute ~ 75% of total fiber

    market

    Polyester is the most important

    Others: nylon, acrylics, and rayon

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    Fiber and Filament Production -

    Spinning For synthetic fibers, spinning= extrusion of polymer

    melt or solution through a spinneret, then drawing andwinding onto a bobbin

    Spinneret = die with multiple small holes, the term isa holdover from methods used to draw and twistnatural fibers into yarn or thread

    Three variations, depending on polymer:

    1. Melt spinning2. Dry spinning

    3. Wet spinning

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    Melt Spinning

    Starting polymer is heated to molten state and

    pumped through spinneret

    Typical spinneret is 6 mm (0.25 in) thick andcontains ~ 50 holes of diameter 0.25 mm (0.010 in)

    Filaments are drawn and air cooled before being

    spooled onto bobbin

    Final diameter wound onto bobbin may be only1/10 of extruded size

    Used for polyester and nylon filaments

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    Melt spinning

    of continuous

    filaments

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    Wet Spinning

    Similar to melt spinning, but polymer is again in

    solution, only solvent is non-volatile

    To separate polymer, extrudate is passed througha liquid chemical that coagulates or precipitates

    the polymer into coherent strands which are then

    collected onto bobbins

    Used for filaments of rayon (regeneratedcellulose)

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    Subsequent Processing of

    Filaments

    Filaments produced by any of the three processes

    are usually subjected to further cold drawing to align

    crystal structure along direction of filament axis Extensions of 2 to 8 are typical

    Effect is to significantly increase tensile strength

    Drawing is done by pulling filament between two

    spools, where winding spool is driven at a fasterspeed than unwinding spool

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    Injection Molding

    Polymer is heated to a highly plastic state and

    forced to flow under high pressure into a mold cavity

    where it solidifies and the moldingis then removed

    from the cavity

    Produces discrete components to net shape

    Typical cycle time 10 to 30 sec, but cycles of

    one minute or more are not uncommon

    Mold may contain multiple cavities, so multiple

    moldings are produced each cycle

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    Injection Molded Parts

    Complex and intricate shapes are possible

    Shape limitations:

    Capability to fabricate a mold whose cavity is thesame geometry as part

    Shape must allow for part removal from mold

    Part size from 50 g (2 oz) up to 25 kg (more than

    50 lb), e.g., automobile bumpers Injection molding is economical only for large

    production quantities due to high cost of mold

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    Injection Molded Parts

    2012 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 5/e

    A collection of

    plastic injection

    molded parts(courtesy of

    George E. Kane

    Manufacturing

    Technology

    Laboratory, Lehigh

    University)

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    Polymers for Injection Molding

    Injection molding is the most widely used molding

    process for thermoplastics

    Some thermosets and elastomers are injectionmolded

    Modifications in equipment and operating

    parameters must be made to avoid premature

    cross-linking of these materials before injection

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    Injection Molding Machine

    Two principal components:

    1. Injection unit

    Melts and delivers polymer melt Operates much like an extruder

    2. Clamping unit

    Opens and closes mold each injection cycle

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    Injection Molding Machine

    A large (3000 ton capacity) injection molding machine (Photo

    courtesy of Cincinnati Milacron)

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    Injection Molding Machine

    Diagram of an injection molding machine, reciprocating screw

    type (some mechanical details are simplified)

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    Injection Unit of Molding Machine

    Consists ofbarrelfed from one end by hopper

    containing supply of plastic pellets

    Inside the barrel is a screwwith two functions:1. Rotates for mixing and heating polymer

    2. Acts as a ram (i.e., plunger) to inject molten plastic

    into mold

    Non-return valve near tip of screw prevents meltfrom flowing backward along screw threads

    Later in cycle ram retracts to its former position

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    Clamping Unit of Molding

    Machine

    Functions:

    1. Holds two halves of mold in proper alignment

    with each other2. Keeps mold closed during injection by applying a

    clamping force sufficient to resist injection force

    3. Opens and closes mold at the appropriate times

    in molding cycle

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    (1) Mold is closed

    Injection Molding Cycle

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    (2) Melt is injected into cavity

    Injection Molding Cycle

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    (3) screw is retracted

    Injection Molding Cycle

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    (4) mold is opened and part is ejected

    Injection Molding Cycle

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    The Mold

    The special tooling in injection molding custom

    designed and fabricated for the part to be produced

    When production run is finished, the mold is replacedwith a new mold for the next part

    Various types of molds for injection molding:

    Two-plate mold

    Three-plate mold Hot-runner mold

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    Details of a

    two-plate mold for

    thermoplastic

    injection molding:(a) closed

    Mold has two

    cavities to produce

    two cup-shaped

    parts with each

    injection shot

    Two-Plate Mold

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    Details of a

    two-plate mold for

    thermoplasticinjection molding:

    (b) open

    Two-Plate Mold

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    Two-Plate Mold Features

    Cavity geometry of part but slightly oversized to

    allow for shrinkage

    Fabricated by machining the mating surfaces oftwo mold halves

    Distribution channel through which polymer melt

    flows from nozzle into mold cavity

    Sprue - leads from nozzle into mold

    Runners - lead from sprue to cavity (or cavities)

    Gates - constrict flow of plastic into cavity

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    More Two-Plate Mold Features

    Ejection system to eject molded part from cavity at

    end of molding cycle

    Ejector pins built into moving half of mold usuallyaccomplish this function

    Cooling system - consists of external pump

    connected to passageways in mold, through which

    water is circulated to remove heat from the hot plastic Air vents to permit evacuation of air from cavity as

    polymer melt rushes in

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    Three-Plate Mold

    Uses three plates to separate parts from sprue and

    runner when mold opens

    Advantages over two-plate mold: As mold opens, runner and parts disconnect and

    drop into two containers under mold

    Allows automatic operation of molding machine

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    Hot-Runner Mold

    Eliminates solidification of sprue and runner by

    locating heaters around the corresponding runner

    channels While plastic in mold cavity solidifies, material in

    sprue and runner channels remains molten, ready to

    be injected into cavity in next cycle

    Advantage: Saves material that otherwise would be scrap in

    the unit operation

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    Injection Molding Machines

    Injection molding machines differ in both injection unitand clamping unit

    Name of injection molding machine is based on thetype of injection unit used

    Reciprocating-screw injection molding machine

    Plunger-type injection molding machine

    Several clamping designs

    Mechanical (toggle)

    Hydraulic

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    Shrinkage

    Polymers have high thermal expansion coefficients,so significant shrinkage occurs during solidificationand cooling in mold

    Plastic Typical Shrinkage, mm/mm (in/in)

    Nylon-6,6 0.020

    Polyethylene 0.025

    Polystyrene 0.004

    PVC 0.005

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    Compensation for Shrinkage

    Dimensions of mold cavity must be larger than

    specified part dimensions:

    Dc= Dp + DpS + DpS2

    where Dc= dimension of cavity; Dp = molded part

    dimension, and S = shrinkage value

    Third term on right hand side corrects for

    shrinkage in the shrinkage

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    Shrinkage Factors

    Fillers in the plastic tend to reduce shrinkage

    Injection pressure higher pressures force more

    material into mold cavity to reduce shrinkage Compaction time - similar effect longer time forces

    more material into cavity to reduce shrinkage

    Molding temperature - higher temperatures lower

    polymer melt viscosity, allowing more material to bepacked into mold to reduce shrinkage

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    Thermoplastic Foam Injection

    Molding

    Molding of thermoplastic parts that possess denseouter skin surrounding lightweight foam center

    Part has high stiffness-to-weight ratio

    Produced either by introducing a gas into moltenplastic in injection unit or by mixing a gas-producingingredient with starting pellets

    A small amount of melt is injected into mold cavity,

    where it expands to fill cavity Foam contacting cold mold surface collapses to form

    dense skin, while core retains cellular structure

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    Injection Molding of Thermosets

    Equipment and operating procedure must be modified to

    avoid premature cross-linking of TS polymer

    Reciprocating-screw injection unit with shorter barrellength

    Temperatures in barrel are relatively low

    Melt is injected into a heated mold, where cross-linking

    occurs to cure the plastic Curing is the most time-consuming step in cycle

    Mold is then opened and part is removed

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    Reaction Injection Molding

    Two highly reactive liquid ingredients are mixed and

    immediately injected into a mold cavity where

    chemical reactions leading to solidification occur RIM was developed with polyurethane to produce

    large automotive bumpers and fenders

    RIM polyurethane parts have a foam internal

    structure surrounded by a dense outer skin Other materials: urea-formaldehyde and epoxies

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    Compression Molding

    Widely used molding process for thermosetting

    plastics

    Also used for rubber tires and polymer matrixcomposite parts

    Molding compound available in several forms:

    powders or pellets, liquid, or preform

    Amount ofcharge must be precisely controlled toobtain repeatable consistency in the molded product

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    Compression Molding

    (1) TS charge is loaded, (2) and (3) compression and

    curing, and (4) part is ejected and removed

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    Molds for Compression Molding

    Simpler than injection molds

    No sprue and runner system in a compression mold

    Process itself generally limited to simpler partgeometries due to lower flow capabilities of TS

    materials

    Mold must be heated, usually by electric resistance,

    steam, or hot oil circulation

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    Compression Molding Materials

    and Products

    Molding materials:

    Phenolics, melamine, urea-formaldehyde,

    epoxies, urethanes, and elastomers Typical compression-molded products:

    Electric plugs, sockets, and housings; pot handles,

    and dinnerware plates

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    Transfer Molding

    TS charge is loaded into a chamber immediatelyahead of mold cavity, where it is heated; pressure isthen applied to force soft polymer into heated mold

    where it cures Two variants:

    Pot transfer molding - charge is injected from a"pot" through a vertical sprue channel into cavity

    Plunger transfer molding

    plunger injects chargefrom a heated well through channels into cavity

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    Pot Transfer Molding

    (1) Charge loaded into pot, (2) soft polymer is pressed into

    mold cavity and cured, and (3) part is ejected

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    Plunger Transfer Molding

    (1) Charge loaded into pot, (2) soft polymer is pressed into

    mold cavity and cured, and (3) part is ejected

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    Compression vs. Transfer

    Molding In both processes, scrap is produced each cycle as

    leftover material, called the cull

    The TS scrap cannot be recovered

    Transfer molding is capable of molding more intricatepart shapes than compression molding

    But not as intricate as injection molding

    Transfer molding is suited to molding with inserts

    Placing a metal or ceramic insert into cavity beforepressing, and the plastic bonds to the insert duringmolding

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    Blow Molding

    Molding process in which air pressure is used to

    inflate soft plastic into a mold cavity

    Important for making one-piece hollow plasticparts with thin walls, such as bottles

    Because these items are used for consumer

    beverages in mass markets, production is

    typically organized for very high quantities

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    Blow Molding Process

    Accomplished in two steps:

    1. Fabrication of a starting tube, called aparison

    2. Inflation of the tube to desired final shape Forming the parison is accomplished by either

    Extrusion or

    Injection molding

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    Extrusion Blow Molding

    (1) Extrusion of parison; (2) parison is pinched at top and

    sealed at bottom around blow pin as the mold closes; (3)

    tube is inflated; and (4) mold is opened

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    Injection Blow Molding

    (1) Parison is injected molded around blowing rod; (2)

    mold is opened and parison is moved to blow mold; (3)

    soft polymer is inflated; and (4) mold is opened

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    Injection Blow Molding

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    An injection blow molded

    bottle on the left and a

    parison for a similar part onthe right (courtesy of

    George E. Kane

    Manufacturing Technology

    Laboratory, Lehigh

    University)

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    Stretch Blow Molding

    Variation of injection blow molding in which blowing rod

    stretches the soft parison for a more favorable

    stressing of polymer than conventional blow molding

    Resulting structure is more rigid, more transparent,

    and more impact resistant

    Most widely used plastic is polyethylene

    terephthalate (PET) which has very low permeability

    and is strengthened by stretch blow molding

    Ideal as container for carbonated beverages

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    Stretch Blow Molding

    (1) Injection molding of parison; (2) stretching; and

    (3) blowing

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    Materials and Products

    in Blow Molding

    Blow molding is limited to thermoplastics

    Materials: high density polyethylene, polypropylene

    (PP), polyvinylchloride (PVC), and polyethyleneterephthalate

    Products: disposable containers for beverages and

    other liquid consumer goods, large shipping drums

    (55 gallon) for liquids and powders, large storage

    tanks (2000 gallon), gasoline tanks, toys, and hulls

    for sail boards and small boats

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    Rotational Molding

    Process uses gravity inside a rotating mold to

    produce a hollow part shape

    Alternative to blow molding for larger parts withmore complex external geometries

    But lower production rates than blow molding

    Mostly for thermoplastic polymers but thermoset

    and elastomer applications are not uncommon Products: boat hulls, sandboxes, buoys, garbage

    cans, large containers and storage tanks

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    Rotational Molding

    Performed on

    three-station

    indexing table:

    (1) unload-load,(2) heat and

    rotate mold, (3)

    cool the mold

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    Thermoforming

    Flat thermoplastic sheet or film is heated and

    deformed into desired shape using a mold

    Heating usually accomplished by radiant electricheaters located on one or both sides of starting

    plastic sheet or film

    Widely used in packaging of products and to

    fabricate large items such as bathtubs, contoured

    skylights, and internal door liners for refrigerators

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    Vacuum Thermoforming

    (1) Flat plastic

    sheet is

    softened by

    heating

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    Vacuum Thermoforming

    (2) The softened sheet

    is placed over a

    concave mold cavity

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    Vacuum Thermoforming

    (3) Vacuum draws

    the sheet into the

    cavity

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    (4) plastic hardens on

    contact with the cold

    mold surface, and the

    part is removed andlater trimmed from the

    web

    Vacuum Thermoforming

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    Negative Molds vs.

    Positive Molds Negative mold has concave cavity, as in previous

    series of diagrams

    Positive mold has convex shape Both types are used in thermoforming

    For positive mold, heated sheet is draped over

    convex form and negative or positive pressure

    forces plastic against mold surface

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    (1) Heated plastic

    sheet is

    positioned above

    the convex mold

    Vacuum Thermoforming using a

    Positive Mold

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    (2) Clamp is lowered

    into position, draping

    the sheet over the mold

    as a vacuum forces thesheet against the mold

    surface

    Vacuum Thermoforming using a

    Positive Mold

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    Materials for Thermoforming

    Only thermoplastics can be thermoformed,

    Sheets of thermosetting or elastomeric polymers

    have already been cross-linked and cannot besoftened by reheating

    Common TP polymers: polystyrene, cellulose

    acetate, cellulose acetate butyrate, ABS, PVC, acrylic

    (polymethylmethacrylate), polyethylene, and

    polypropylene

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    Applications of Thermoforming

    Thin films: blister packs and skin packs for packagingcommodity products such as cosmetics, toiletries,small tools, and fasteners (nails, screws, etc.)

    For best efficiency, filling process to containerizeitem(s) is immediately downstream fromthermoforming

    Thicker sheet stock: boat hulls, shower stalls,

    advertising displays and signs, bathtubs, certain toys,contoured skylights, internal door liners forrefrigerators

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    Casting

    Pouring liquid resin into a mold, using gravity to fill

    cavity, where polymer hardens

    Both thermoplastics and thermosets are cast Thermoplastics: acrylics, polystyrene,

    polyamides (nylons) and PVC

    Thermosets: polyurethane, unsaturated

    polyesters, phenolics, and epoxies Simple mold

    Suited to low quantities

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    Polymer Foam

    A polymer-and-gas mixture that gives the material a

    porous or cellular structure

    Most common polymer foams: polystyrene(Styrofoam, a trademark) and polyurethane

    Other polymers: natural rubber ("foamed rubber")

    and polyvinylchloride (PVC)

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    Properties of a Foamed Polymer Low density

    High strength per unit weight

    Good thermal insulation Good energy absorbing qualities

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    Classification of Polymer Foams

    Elastomeric - matrix polymer is a rubber, capable of

    large elastic deformation

    Flexible - matrix is a highly plasticized polymer suchas soft PVC

    Rigid - polymer is a stiff thermoplastic such as

    polystyrene or a thermoset such as a phenolic

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    Applications of Polymer Foams

    Characteristic properties of polymer foams, and the

    ability to control elastic behavior by selection of base

    polymer, make these materials suitable for certain

    applications

    Applications: hot beverage cups, heat insulating

    structural materials, cores for structural panels,

    packaging materials, cushion materials for furniture

    and bedding, padding for automobile dashboards,and products requiring buoyancy

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    Two polymer foam structures: (a) closed cell and

    (b) open cell

    Polymer Foam Structures

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    Extrusion of Polystyrene Foams

    Polystyrene (PS) is a thermoplastic polymer

    A physical or chemical blowing agent is fed into

    polymer melt near die end of extruder barrel Thus, extrudate consists of expanded polymer

    Products: large sheets and boards that are

    subsequently cut to size for heat insulation panels

    and sections

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    Molding Processes for

    Polystyrene Foams

    Expandable foam molding

    Molding material consists of prefoamed

    polystyrene beads Beads are fed into mold cavity where they are

    further expanded and fused together to form the

    molded product

    Products: hot beverage cups

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    Shaping of Polyurethane Foams

    Polyurethane can be thermosetting, elastomer orthermoplastic (less common)

    Polyurethane foam products are made in a one-step

    process in which the two liquid ingredients are mixedand immediately fed into a mold or other form

    Polymer is synthesized and part geometry iscreated at the same time

    Shaping processes for polyurethane foam: Spraying

    Pouring

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    Product Design Guidelines:

    General

    Strength and stiffness

    Plastics are not as strong or stiff as metals

    Avoid applications where high stresses will beencountered

    Creep resistance is also a limitation

    Strength-to-weight ratios for some plastics are

    competitive with metals in certain applications

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    Product Design Guidelines:

    General

    Impact Resistance

    Capacity of plastics to absorb impact is generally

    good; plastics compare favorably with most metals

    Service temperatures

    Limited relative to metals and ceramics

    Thermal expansion

    Dimensional changes due to temperature changesmuch more significant than for metals

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    Product Design Guidelines:

    General

    Many plastics are subject to degradation from

    sunlight and other forms of radiation

    Some plastics degrade in oxygen and ozone

    atmospheres

    Plastics are soluble in many common solvents

    Plastics are resistant to conventional corrosion

    mechanisms that afflict many metals

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    Product Design Guidelines:

    Extrusion

    Wall thickness

    Uniform wall thickness is desirable in an extruded

    cross section

    Variations in wall thickness result in non-uniform

    plastic flow and uneven cooling which tend to

    warp extrudate

    G

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    Product Design Guidelines:

    Extrusion

    Hollow sections

    Hollow sections complicate die design and plastic

    flow

    Desirable to use extruded cross sections that are

    not hollow yet satisfy functional requirements

    P d t D i G id li

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    Product Design Guidelines:

    Extrusion

    Corners

    Sharp corners, inside and outside, should be

    avoided in extruded cross sections

    They result in uneven flow during processing and

    stress concentrations in the final product

    P d t D i G id li

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    Product Design Guidelines:

    Moldings

    Economic production quantities

    Each part requires a unique mold, and the moldfor any molding process can be costly, particularly

    for injection molding Minimum production quantities for injection

    molding are usually ~ 10,000 pieces

    For compression molding, minimum quantities are1000 parts, due to simpler mold designs

    Transfer molding lies between injection moldingand compression molding

    P d t D i G id li

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    Product Design Guidelines:

    Moldings

    Part complexity

    An advantage of plastic molding is that it allows

    multiple functional features to be combined into

    one part

    Although more complex part geometries mean

    more costly molds, it may nevertheless be

    economical to design a complex molding if the

    alternative involves many individual parts thatmust be assembled

    P d t D i G id li

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    Product Design Guidelines:

    Moldings

    Wall thickness

    Thick cross sections are wasteful of material, more

    likely to cause warping due to shrinkage, and take

    longer to harden

    Reinforcing ribs

    Achieves increased stiffness without excessive

    wall thickness

    Ribs should be made thinner than the walls they

    reinforce to minimize sink marks on outside wall

    P d t D i G id li

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    Product Design Guidelines:

    Moldings

    Corner radii and fillets

    Sharp corners, both external and internal, are

    undesirable in molded parts

    They interrupt smooth flow of the melt, tend to

    create surface defects, and cause stress

    concentrations in the part

    Holes

    Holes are quite feasible in plastic moldings, but

    they complicate mold design and part removal

    P d t D i G id li

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    Product Design Guidelines:

    Moldings

    Draft

    A molded part should be designed with a draft on

    its sides to facilitate removal from mold

    Especially important on inside wall of a

    cup-shaped part because plastic contracts against

    positive mold shape

    Recommended draft:

    For thermosets, ~ 1/2 to 1

    For thermoplastics, ~ 1/8 to 1/2

    P d t D i G id li

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    Product Design Guidelines:

    Moldings

    Tolerances

    Although shrinkage is predictable under closely

    controlled conditions, generous tolerances are

    desirable for injection moldings because of

    Variations in process parameters that affect

    shrinkage

    Diversity of part geometries encountered